In phonetics, nasalization (or nasalisation) is the production of a sound while the velum is lowered, so that some air escapes through the nose during the production of the sound by the mouth. An archetypal nasal sound is [n].

In the International Phonetic Alphabet nasalization is indicated by printing a tilde diacritic Template:Unichar above the symbol for the sound to be nasalized: [ã] is the nasalized equivalent of [a], and [ṽ] is the nasalized equivalent of [v]. An older IPA subscript diacritic [ą], called an ogonek, is still seen, especially when the vowel bears tone marks that would interfere with the superscript tilde. For example, [ą̄ ą́ ą̀ ą̂ ą̌] are more legible in most fonts than [ã̄ ã́ ã̀ ã̂ ã̌].

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Nasal vowels are found in many European languages, such as French, Portuguese, Breton, Polish. In these, as well as and languages found in other language families outside Europe, nasal vowels contrast with oral vowels. Many languages, however, only have oral vowels.

There are occasional cases where vowels show contrasting degrees of nasality[citation needed].

By far the most common nasalized sounds are nasal stops such as [m], [n] or [ŋ]. They may be called stops because airflow through the mouth is blocked, though air flows freely through the nose. Their non-nasal articulatory counterparts are the oral stops.

Nasalized versions of other consonant sounds also exist, though they are much rarer than either nasal stops or nasal vowels. Some of the South Arabic languages have phonemic nasalized fricatives, such as /z̃/, which sounds something like a simultaneous [n] and [z]. The sound written ⟨r⟩ in Mandarin has an odd history; for example, it has been borrowed into Japanese as both [z] and [n]. It seems likely that it was once a nasalized fricative, perhaps a palatal [ʝ̃]. In the Hupavelar nasal/ŋ/, the tongue often does not make full contact, resulting in a nasalized approximant, [ɰ̃]. This is cognate with a nasalized palatal approximant[ȷ̃] in other Athabaskan languages. In Umbundu, phonemic /ṽ/ contrasts with the (allophonically) nasalized approximant [w̃], and so is likely to be a true fricative rather than an approximant.Template:Elucidate In Old and Middle Irish, the lenited ⟨m⟩ was a nasalized bilabial fricative.[1]

Besides nasalized oral fricatives, there are true nasal fricatives, called nareal fricatives, sometimes produced by people with speech defects. That is, the turbulence in the airflow characteristic of fricatives is produced not in the mouth but in the nasal cavity. A tilde plus trema diacritic is used for this in the Extensions to the IPA: [n͋] is an alveolar nareal fricative, with no airflow out of the mouth, while [v͋] is an oral fricative (a [v]) with simultaneous nareal frication. No known natural language makes use of nareal consonants.

Vowels assimilate to surrounding nasal consonants in many languages, such as Thai, creating nasal vowel allophones. Some languages exhibit a nasalization of segments adjacent to phonemic or allophonic nasal vowels, such as Apurinã. Contextual nasalization can lead to the addition of nasal vowel phonemes to a language.[2] This happened in French, where most final consonants disappeared, but where in the case of final nasals, the preceding vowels became nasal, introducing a new distinction into the language. An example where this happened is vin blancTemplate:IPA-fr ('white wine'), ultimately from Latin vinum and blancum.